The present invention relates to a semiconductor exposing system for projecting a fine pattern to a semiconductor wafer and, more particularly, to a system for correcting an influence by a fluctuation of a wavelength of a short wavelength light source.
In recent years, high density of semiconductor devices have been developed. At present, the width of the thinnest line which is required in semiconductor devices is on the order of submicron. A light source of a short wavelength such as far ultraviolet rays is required to accurately project such a thin line onto a semiconductor wafer while keeping a desired resolution. Nowadays, a g-line (436 nm) or i-line (365 nm) of the spectrum of a mercury lamp is used. As the wavelength becomes short, the minimum line width which can be projected onto the wafer can be narrowed. Further, an excimer laser is known as a light source of a short wavelength. An oscillating wavelength of the excimer laser can be changed by changing the kind of oscillating medium gas. The oscillating media by which high power can be stably obtained are xenon chloride (XeCl) of a wavelength of 308 nm and krypton fluoride (KrF) of a wavelength of 248 nm.
Only synthetic quartz or calcium fluoride (CaF.sub.2) can be at present used as a lens material which can transmit such a light of a short wavelength. Further, it is desirable to make a lens using only synthetic quartz in terms of the uniformity and stability of the material.
A spectrum width of the oscillating output light of the excimer laser is 0.3 nm. To obtain high enough resolution to expose a mask pattern onto a wafer using the light source of such a spectrum width, the chromatic aberration must be corrected. To correct the chromatic aberration, at least two kinds of materials having different refractive indices and Abbe's Numbers are needed. However, only one kind of synthetic quartz is available as a practical lens material which transmits the light of the excimer laser, so that it is impossible to correct the chromatic aberration.
There has been proposed a method whereby the correction of the chromatic aberration becomes unnecessary by narrowing the band of the output light of the excimer laser. The narrow spectrum can be selectively taken out by inserting a wavelength tuning device such as a combination of prisms and a diffraction grating into a laser resonator. An excimer laser exposing apparatus using an etalons as a wavelength tuning device is disclosed in Victor Pol et al., "Excimer laser-based lithography" in Proceedings of SPIE, Optical Microlithography, V(1986).
To obtain a narrow band laser beam having a desired central wavelength, the geometrical position of the wavelength tuning device in the resonator must be accurately held. However, the device is deformed by a change in ambient temperature of the wavelength tuning device, so that the center wavelength of the output beam is shifted. The lens for reducing a mask pattern and projecting onto a wafer is designed such that its optical performance is made effective at a certain special wavelength. Therefore, if the center wavelength of the light source is shifted, the projected pattern becomes dim or a reduction ratio changes, so that the size of the projected pattern varies. These items are large contributors to a deterioration of the manufacturing yield of the semiconductor devices. Therefore, in a projection type exposing system using a wavelength tuning device in a laser light source, a change in parameter of the optical system, i.e., image forming position which is caused due to a fluctuation in wavelength needs to be corrected.